Im considering buying a new sthil ms 251. any opinions or comments ?
What're you gonna use it for? For occasional use in wood under 14" it'll be great, though I'd personally find an ms250 instead if you're set on a new stihl saw for that price. 250s are still on the shelf at dealerships, and they're cheaper than the 251. The 250 is a simpler design, and for a cheap saw, they hold up very well.
Both the 250 and 251 are "homeowner" type saws with a plastic clamshell design, rather than the magnesium split case design of a pro grade saw. The pro models to look at in this size are the 241 and 261. I've never used a 241, but people rave about them. My 261 is an awesome saw. Lightweight, tons of power for a 50cc saw.
I know I said I'd take a 250 instead, but I'd actually go a different route personally. I'd find a used stihl 026 or 260, which are pro grade predecessors to the 261. I also have an 026, and that thing will cut circles around a 251. Another option in that size range would be the Husky 346/351/353 family, which are also older models, or the Dolmar 421, which is a little smaller than the 026/351 saws, but is a great little saw that starts super easy and is built more like a pro saw than the 250/251 for about the same price. The Dolmar 5105 is a 50cc, and is a great saw, but a little pricier than the 251. Still a lot cheaper than a 261, though.
Good luck in your decision, and let us know what you end up with. Also, if this is your first saw (or you don't already have them) leave room in the budget for chainsaw chaps. Proper PPE is extremely important when running saws.